An electronic oven is a device in which cooking is effected by making use of the nature that an irradiated microwave is absorbed by molecules of water or the like contained in an article to be cooked, and it has a merit that generally cooking can be achieved in a short period of time. On the other hand, it cannot scorch food surfaces as is the case with external heating as by an oven, a gas range, an electric heater or the like.
In order to overcome the above-mentioned shortcoming, a heat-generating body or a heat-generating container capable of scorching foods by making use of substance which generates heat by absorbing a microwave such as ferrite, SiC, metal, barium titanate, etc., has been devised, and a sintered body of ferrite, silicon carbide or the like, a pottery having the sintered body assembled therein, and furthermore, a body formed by applying powder of these materials to a base material as a coating film, have been devised.
However, these heat-generating bodies involve many problems such that its heat-generating property is insufficient, they cannot withstand thermal shocks caused by abrupt heat-generation, and they are expensive and heavy in weight.
On the other hand, while a heat-generating sheet formed by applying metal vapor deposition of aluminium or the like onto a heat-resisting paper sheet, a heat-resisting resin film or the like has been devised, it has a shortcoming that a stable heat-generating quantity can be hardly assured because of the fact that it is necessary to make the thickness of vapor deposition considerably thin. It is hard to uniformly control the thickness due to its thin film state, and its heat-generating property would largely vary in the event that the thickness of the vapor deposition film should change.
A microwave-absorptive heat-generating body is formed of substances having a heat-resisting property in view of its function. A principal method for manufacture thereof includes a method of forming a conductive thin film of Al, SnO.sub.2, etc. on a surface of a heat-resisting base material through vapor-deposition; a method of obtaining the heat-generating body by sintering powder having a microwave-absorptive heat-generating property such as ferrite, SiC, BaTiO.sub.3, etc.; and a method of fixedly securing powder having a microwave-absorptive heat-generating property onto a surface of a heat-resisting base material by means of a heat-resisting organic bonding agent.
However, the above-mentioned heat-generating bodies produced through vapor deposition and/or sintering necessitate a high temperature at the time of manufacture and also result in a high cost in view of installation or the like. Also, the method of fixedly securing material having a microwave-absorptive heat-generating property by means of an organic bonding agent is limited with respect to its heat-resisting temperature.
In addition, although a method of fixedly securing the material by heating an inorganic bonding agent or adding a hardening agent to the inorganic bonding agent can be conceived in order to resolve the above-mentioned problems, even in such method, in the case of necessitating to heat, rise of energy and installation costs result, while in the case of adding a hardening agent, degradation of the working efficiency caused by decrease in the available time result. In either case, neither method is suitable to the case where mass-production is required.
The present invention has been worked out in view of the above-mentioned point, and one object of the present invention is to provide a sheet-like microwave-absorptive heat-generating body which is light in weight, flexible, excellent in a heat-generating property, and moreover, cheap.
Another object of the present invention is to provide a method for forming a microwave-absorptive heat-generating layer on a heat-resisting base material by making use of an inorganic bonding agent at a low temperature, and moreover, under a sufficient available time.